1. Field of the Invention
The present invention relates to a hydraulic control means for an automatic transmission comprising a fluid torque converter and a gear system used for vehicles such as an automobile and the like.
A fluid automatic transmission is, as is well known, constructed in such a way that the shift is automatically controlled relating to the vehicle speed and throttle opening. The vehicle speed is transformed into a hydraulic signal proportional to the vehicle speed by a governor valve, and the throttle opening is transformed into a hydraulic signal proportional to it by a throttle valve. These two hydraulic signals are led to more than one or two shift valves, and shift control is completed by engaging brakes or clutches attached to a gear system. Such fluid automatic transmission has such a characteristic that shift transition is normally controlled on a shift line related to the throttle opening and vehicle speed, and this shift line operates such that vehicle speed is higher as the throttle opening is larger. This hydraulic control means is constructed in such a way that a further hysteresis effect is given to shift characteristic, in order to stabilize the operation of an automatic transmission near the shift point and in case of down-shift, the down-shift is performed by vehicle speed lower than vehicle speed at the up-shift if the throttle opening is the same. It is further required for the shift characteristic that in order to prevent the shock at the shift transition, the width of hysteresis at high throttle opening is made as large as possible, and further in order to make the range for use at a high speed stage large, the width of hysteresis at low throttle opening is made as small as possible.
2. Description of the Prior Arts
Hitherto, there difficulties have arisen in that that shift shock is liable to be caused when the width of hysteresis is small, and especially the shift shock does not cease to be caused in case of the running on a severe road in going up and down at overdrive (4-speed) or 3-speed in the relation to the shift characteristic at going-up of a slope. A method to improve a conventional shift valve has been devised to give a hysteresis effect to the shift characteristic, in order to stabilize the operation of an automatic transmission near the shift point by noticing these difficult points as disclosed in U.S. Pat. No. 4,143,563 and so on, but this method had the defect that the width of hysteresis becomes narrow partially in some conditions and shift hunting is liable to be caused, because of sudden change in the width of hysteresis at a certain opening of the accelerator.